Ni-Ti-Cu shape memory alloy electrothermal actuator element

ABSTRACT

An Ni—Ti—Cu shape memory alloy electrothermal actuator element that recovers its original shape by electrical heating, having a wire diameter of 0.5 mm or less, comprising an Ni—Ti—Cu shape memory alloy wire which contains 49.0 to 51.0 at % of Ti, and 5.0 to 12.0 at % of Cu, with the balance being made of Ni, wherein the actuator element has a deterioration rate of shape strain recovery of 0.5% or less after repeating desired times of shape recovery movement.

FIELD

[0001] The present invention relates to an Ni—Ti—Cu shape memory alloy electrothermal actuator element.

BACKGROUND

[0002] A shape memory alloy actuator element is, for example, a linear wire that memorizes its original length. The element can repeat a reciprocating movement to cause a prescribed strain at room temperature, and to recover to its original memorized length at a temperature of Af (a finishing temperature of reverse transformation) or higher, under a weight load.

[0003] Meanwhile, an Ni—Ti shape memory alloy wire has been used as the above-described conventional shape memory alloys. However, there is a problem that this material results some degree of permanent strain after repeating the above-described reciprocating movements many times, thereby causing a certain degree of strain at room temperature that finally leads the material to failure in recovery of its original memorized length at a temperature of Af or higher (hereinafter, this phenomenon is referred to as increasing of a deterioration rate of shape strain recovery).

[0004] To solve the problem described above, alloy wires having a small deterioration rate of shape strain recovery have been sought, and thus an Ni—Ti—Cu shape memory alloy wire has been proposed (JP-A-2-116786 (“JP-A” means unexamined published Japanese patent application)). However, this wire has the problem that its deterioration rate of shape strain recovery cannot be sufficiently reduced, and that its response speed is still slow, since heating of actuator elements using this alloy wire to recover its original shape is operated through water or air as a medium.

SUMMARY

[0005] The present invention is an Ni—Ti—Cu shape memory alloy electrothermal actuator element that recovers its original shape by electrical heating, having a wire diameter of 0.5 mm or less, which actuator element is composed of an Ni—Ti—Cu shape memory alloy wire which contains 49.0 to 51.0 at % of Ti, and 5.0 to 12.0 at % of Cu, with the balance being made of Ni, wherein the actuator element has a deterioration rate of shape strain recovery of 0.5% or less after repeating desired times of shape recovery movement.

[0006] Other and further features and advantages of the invention will appear more fully from the following description, take in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1(A) and FIG. 1(B) each are a schematic view illustrating an electrical heating fatigue testing machine; FIG. 1(A) represents the state when electrical heating, and FIG. 1(B) represents the state when standing to cool.

[0008]FIG. 2 is a schematic view illustrating deterioration rates of shape strain recovery α and β, and a shape recovery strain rate γ.

DETAILED DESCRIPTION

[0009] According to the present invention, there is provided the following means:

[0010] (1) An Ni—Ti—Cu shape memory alloy electrothermal actuator element that recovers its original shape by electrical heating, having a wire diameter of 0.5 mm or less, comprising an Ni—Ti—Cu shape memory alloy wire which contains 49.0 to 51.0 at % of Ti, and 5.0 to 12.0 at % of Cu, with the balance being made of Ni, wherein the actuator element has a deterioration rate of shape strain recovery of 0.5% or less after repeating desired times of shape recovery movement;

[0011] (2) The Ni—Ti—Cu shape memory alloy electrothermal actuator element according to item (1), wherein the Cu content of the Ni—Ti—Cu shape memory alloy is 6.0 to 8.0 at %;

[0012] (3) The Ni—Ti—Cu shape memory alloy electrothermal actuator element according to item (1) or (2), wherein the Ni—Ti—Cu shape memory alloy wire is subjected to a shape memory heat-treatment at a temperature in the range from 400 to 600° C.; and

[0013] (4) An Ni—Ti—Cu shape memory alloy electrothermal actuator element, wherein the wire in the actuator element according to any one of items (1) to (3) is subjected to repeating cycles of heating to a temperature of Af or higher with applying a weight load corresponding to 10 to 30% of a breaking load of the wire, and cooling to a temperature of Mf (a finishing temperature of Martensitic transformation) or lower, and wherein the deterioration rate of shape strain recovery after repeating desired times of shape recovery movement is 0.2% or less.

[0014] The actuator element of the present invention composed of an Ni—Ti—Cu shape memory alloy. Cu, which is one of elements of the alloy, functions for reducing the deterioration rate of shape strain recovery and accelerating the response speed.

[0015] The Cu content is restricted within the range of 5.0 to 12.0 at % because a sufficient effect cannot be attained at the Cu content of less than 5.0 at %, and on the other hand, in case of the Cu content of more than 12.0 at %, the workability of the alloy becomes poor, further the shape recovery strain rate (the difference between the strain rate when heating, and the strain rate when cooling, under a weight load) is reduced. Cu content of 6.0 to 8.0 at % is particularly preferable for further increasing the shape recovery strain rate and stable recovering the original shape of the resultant actuator element.

[0016] The Ti content is restricted within the range of 49.0 to 51.0 at % because workability of the arroy becomes poor outside the above-described range of Ti content.

[0017] In the present invention, a heating for shape recovery is conducted by electrical heating, this is because according to the electrical heating, the heating speed is high (a response speed is high), heating operation is simple, the heating speed can be freely controlled by changing the electrothermal current, and the like.

[0018] On the other hand, in the electrical heating, the temperature distribution tends to be ununiform because heating rate is so fast. Accordingly, the fatigue of the wire is easy to occur. However, this defect was solved in the present invention by thinning down the wire diameter to 0.5 mm or less. When the wire diameter is thinned down to 0.5 mm or less, it permits the deterioration rate of shape strain recovery to be reduced and the response time to be shortened since the heating and cooling rates of the shape memory alloy wire are increased.

[0019] The Ni—Ti—Cu shape memory alloy wire that can be used in the actuator element of the present invention, is used as a liner shape, or another arbitrary shape such as a coiled shape. A linear actuator element permits a current density, a temperature distribution, and a stress distribution, when electrical heating, to be uniform, because of the simple shape thereof, thereby allowing the actuator to be widely designed.

[0020] The actuator element of the present invention is used by moving within a narrow strain width (within a narrow temperature hysteresis width) close to an elasticity range, and amplifying the movement. By that, the deterioration rate of shape strain recovery can be further reduced, the lifetime of the actuator element can be prolonged, and reproducibility of the movement can be improved. The amplification of the above-described movement is attained, for example, by the coil shape-actuator element.

[0021] The Ni—Ti—Cu shape memory alloy wire to be used in the present invention can be manufactured in a usual manner by sequentially applying hot-working, cold-drawing, shaping, and shape-memory-heat-treatment, in this order, to an ingot of the Ni—Ti—Cu shape memory alloy. Intermediate annealing can be appropriately applied in the cold-drawing divided into two-steps. A final cold-drawn ratio of 15 to 60% is preferable since the shape recovery strain rate increases in this range of the ratio. The temperature for the above-described shape memory heat-treatment is preferably in the range of 400 to 600° C., since a sufficient shape recovery strain rate may not be obtained in some cases when the heat-treatment temperature is too low or too high.

[0022] The deterioration rate of shape strain recovery of the Ni—Ti—Cu alloy actuator element according to any one of the above-described items (1) to (3) is somewhat large at an initial stage of strain. The present invention according to the above-described item (4) is an actuator element, wherein the above initial somewhat large deterioration rate is previously alleviated, by applying a pre-treatment, in which the above-described Ni—Ti—Cu shape memory alloy wire is “subjected repeatedly to heating and cooling processes to heat the wire to a temperature of Af or higher, and to cool the wire to a temperature of Mf or lower, under a weight load”, and thereby the deterioration rate of shape strain recovery of the actuator element when it is used is improved to 0.2% or less.

[0023] When the weight load in the above-described pre-treatment is so large that the wire receives a plastic deformation, the wire is largely damaged, and the shape recovery strain rate is reduced. On the other hand, when the weight load is too small, a sufficient effect by the pre-treatment cannot be obtained. Accordingly, it is preferable to apply the above-described weight load corresponding to 10 to 30%, particularly preferably 15 to 25%, of the wire's breaking load.

[0024] The actuator element can be heated to a temperature of Af or higher in the above-described pre-treatment, according to an arbitrary method such as electrical heating or heating in a furnace. Since a too high heating temperature damages the wire largely, and reduces the shape recovery strain rate, the preferable temperature is in the range of {Af+(10 to 50)}° C. The wire may be non-forcibly cooled (stood to cool) at a temperature of Mf or lower sufficiently, but it may be forced to cool by, for example, blowing air, since the wire has a small diameter.

[0025] Since the actuator element of the present invention is composed of the Ni—Ti—Cu shape memory alloy wire having a small diameter that can recover its original shape by electrical heating, it has a small deterioration rate of shape strain recovery and also has a rapid response time. According to the actuator element of a preferable embodiment of the present invention, wherein the Cu content is further controlled to be in the preferable range of 6.0 to 8.0 at %, or wherein the temperature for the shape memory heat-treatment is controlled within the preferable range of 400 to 600° C., or wherein the wire for the actuator element is pre-treated by repeatedly heating/cooling a plurality of times to heat to a temperature of Af or higher and to cool to a temperature of Mf or lower, with applying a weight load corresponding to 10 to 30% of the breaking load, thereby the deterioration rate of shape strain recovery can be further reduced. Accordingly the present invention exhibits industrial remarkable effects.

[0026] The present invention will be explained in more detail referring to the following examples, but the invention is not limited thereto.

EXAMPLE Example 1

[0027] Hot-working was applied to an ingot of a Ni—Ti—Cu alloy having a composition defined in the present invention, as shown in Table 1, then a wire of the resultant Ni—Ti—Cu alloy was manufactured by applying cold-drawing, with an appropriate intermediate annealing between the steps of the cold-drawing. The diameter of the thus-obtained wire was adjusted to 0.05, 0.20, or 0.50 mm. The final cold-drawing ratio was 40% in each cases.

Comparative Example 1

[0028] The wires of Ni—Ti—Cu alloys were manufactured in the same manner as in Example 1, except that ingots of Ni—Ti—Cu alloys with compositions outside the definition in the present invention, as shown in Table 1, were used, respectively (Sample Nos. 8 and 10).

Comparative Example 2

[0029] The wire of a Ni—Ti—Cu alloy was manufactured in the same manner as in Example 1, except that the diameter of the wire was adjusted to 0.80 mm (Sample No. 9).

Comparative Example 3

[0030] The wires of Ni—Ti alloys were manufactured in the same manner as in Example 1, except that ingots of Ni—Ti alloys with compositions, as shown in Table 1, were used, respectively (Sample Nos. 6 and 7).

[0031] The Sample No. 10 wire, among the wires obtained in Example 1 and Comparative examples 1 to 3, contained so much Cu that workability was poor, to fail in drawing to a wire with a desired diameter of 0.5 mm.

[0032] The wires (Sample Nos. 1 to 9) other than the Sample No. 10 wire, were applied to the shape memory heat-treatment under the conditions as shown in Table 1, to make the wires memorize their original lengths. Then, the wires were subjected to cycle tests of electrical heating and standing to cool, to determine a deterioration rate of strain α when electrical heating and a deterioration rate of strain β when standing to cool, using an electrical heating fatigue testing machine. Further, a shape recovery strain rate γ thereof at initial stage is also determined.

[0033] The above-described deterioration rate α is the strain rate when electrical heating after 1,000 cycles of the tests, and the above-described deterioration rate β is a value determined by subtracting the strain rate when standing to cool at the initial stage from the strain rate when standing to cool after 1,000 cycles of the test, and the above-described shape recovery strain rate γ is the strain rate when standing to cool at the initial stage (see FIG. 2). Further, the above-described deterioration rates α and β were almost saturated after 1,000 cycles of the test, respectively.

[0034] The above-described electrical heating fatigue testing machine 1 has a construction, in which the both ends of a shape memory alloy wire (actuator element) 2 to be tested are held with pressure connection terminals 3. As shown in FIGS. 1(A) and 1(B), one of the pressure connection terminals 3 is connected to a SUS sleeve shaft 5 through a holder 4, and tension is applied to the shape memory alloy wire 2 by pulling the sleeve shaft 5 with a bias spring 6.

[0035] The shape memory alloy wire 2 recovers its originally memorized length, against the tension from the bias spring 6, as shown in FIG. 1(A), by heating to a temperature of Af or higher. When the wire is cooled to a temperature of Mf or lower, the mechanical strength of the wire is reduced, and the wire occurs strain (elongated) by yielding to the tension from the bias spring 6, as shown in FIG. 1(B). The shape memory alloy wire 2 is electrically heated with an electrothermal device (not shown).

[0036] The test results are shown in Table 2, together with the cycle test conditions. TABLE 1 Shape memory heat- Wire treatment condition Sample Alloy Ti Cu diameter Temperature Time Classification No. No. (at %) (at %) Ni (mm) (° C.) (minute) Example 1 a 50.5 7.0 Balance 0.20 500 0.5 according to 2 b 50.5 9.0 Balance 0.05 500 0.5 this 3 b 50.5 9.0 Balance 0.20 500 0.5 invention 4 b 50.5 9.0 Balance 0.50 500 0.5 5 c 50.5 11.0 Balance 0.20 500 0.5 Comparative 6 f 50.0 0.0 Balance 0.05 500 0.5 example 7 f 50.0 0.0 Balance 0.20 500 0.5 8 g 50.5 3.0 Balance 0.20 500 0.5 9 b 50.5 9.0 Balance 0.80 500 0.5 10 h 50.5 13.0 Balance Impossible to work to a wire diameter of 0.5 mm.

[0037] TABLE 2 Deterioration rate Cycle test condition of shape recovery Electro- strain Shape thermal Time for When When recovery Weight current standing heated stood strain Sample Alloy load (A) × time to cool electrically to cool rate γ Classification No. No. (MPa) (second) (second) α (%) β (%) (%) Example 1 a 175 0.50 × 5 15 0.45 0.33 4.02 according to 2 b 275 0.10 × 5 5 0.09 0.01 4.23 this invention 3 b 175 0.50 × 5 15 0.13 0.02 3.78 4 b 175 2.00 × 10 25 0.35 0.43 4.15 5 c 175 0.50 × 5 15 0.06 0.03 3.51 Comparative 6 f 275 0.10 × 5 5 2.27 0.80 4.86 example 7 f 175 0.50 × 5 15 2.52 2.14 4.32 8 g 175 0.50 × 5 15 0.89 0.71 4.20 9 b 175 3.00 × 20 60 0.76 0.53 3.91

[0038] As is apparent from the results in Table 2, the deterioration rates of shape strain recovery α and β were as small as 0.5% or less in the sample Nos. 1 to 5 of examples according to the present invention, even after repeating 1,000 times of the shape recovery movement. In other words, the wires could almost recover their originally memorized length after 1,000 cycles of the test, indicating good shape recovery abilities. The sample No. 1 had a larger shape recovery strain rate γ, as compared with the sample Nos. 3 and 5, since the Cu content was optimum in the sample No. 1.

[0039] On the contrary, both the sample Nos. 6 and 7 were poorly large in the deterioration rates of shape strain recovery since Cu was not contained in these samples. The deterioration rates of shape strain recovery α and β each were poorly large in sample Nos. 8 and 9, respectively, because the Cu content in the sample No. 8 was too small and the wire of the sample No. 9 was too large in diameter.

Example 2

[0040] The wire of the sample No. 1 that was prepared in the same manner as in Example 1 was subjected to a pre-treatment by repeating ten times of cycles of heating in a furnace at a temperature of 110° C. (Af+25° C.) for five seconds, with applying a weight load of 300 MPa (22% of the breaking load), and then standing to cool at room temperature for five seconds. Then, the electrical heating fatigue test was conducted in the same manner as in the Example 1. As a result, the deterioration rates α and β were reduced from 0.45% to 0.1%, and from 0.33% to 0.02%, respectively, showing large improvements of the shape recovery strain rates.

Example 3

[0041] The time required for deformation when standing to cool (the response time) in the electrical heating fatigue test was determined with respect to the sample Nos. 2 and 3 (Ni—Ti—Cu shape memory alloys) and the sample Nos. 6 and 7 (Ni—Ti shape memory alloys) that were prepared in the same manner as in Example 1. The results are shown in Table 3. TABLE 3 Wire Deformation Sample Alloy diameter time Classification No. No. (mm) (second) Example 2 b 0.05 0.4 according to 3 b 0.20 11.9  this invention Comparative 6 f 0.05 0.6 example 7 f 0.20 14.1 

[0042] As is apparent from the results in Table 3, the response speeds of the samples according to the present invention (the Sample Nos. 2 and 3) were much faster than those of the samples of the comparative examples (the Samples Nos. 6 and 7) in case of both 0.05 mm and 0.20 mm of the wire diameter. This is because the temperature hysteresis was quite small in the examples according to the present invention, as a result of adding Cu.

[0043] Having described our invention as related to the present embodiments, it is our intention that the invention not be limited by any of the details of the description, unless otherwise specified, but rather be construed broadly within its spirit and scope as set out in the accompanying claims. 

What is claimed is:
 1. An Ni—Ti—Cu shape memory alloy electrothermal actuator element that recovers its original shape by electrical heating, having a wire diameter of 0.5 mm or less, comprising an Ni—Ti—Cu shape memory alloy wire which contains 49.0 to 51.0 at % of Ti, and 5.0 to 12.0 at % of Cu, with the balance being made of Ni, wherein the actuator element has a deterioration rate of shape strain recovery of 0.5% or less after repeating desired times of shape recovery movement.
 2. The Ni—Ti—Cu shape memory alloy electrothermal actuator element as claimed in claim 1, wherein the Cu content of the Ni—Ti—Cu shape memory alloy is 6.0 to 8.0 at %.
 3. The Ni—Ti—Cu shape memory alloy electrothermal actuator element as claimed in claim 1, wherein the Ni—Ti—Cu shape memory alloy wire is heat-treated to achieve memorization of its original shape at a temperature in the range from 400 to 600° C.
 4. The Ni—Ti—Cu shape memory alloy electrothermal actuator element as claimed in claim 2, wherein the Ni—Ti—Cu shape memory alloy wire is heat-treated to achieve memorization of its original shape at a temperature in the range from 400 to 600° C.
 5. An Ni—Ti—Cu shape memory alloy electrothermal actuator element, wherein the wire in the actuator element as claimed in claim 1 is subjected to repeating cycles of heating to a temperature of Af or higher with applying a weight load corresponding to 10 to 30% of a breaking load of the wire, and cooling to a temperature of Mf (a finishing temperature of Martensitic transformation) or lower, and wherein the deterioration rate of shape strain recovery after repeating desired times of shape recovery movement is 0.2% or less.
 6. An Ni—Ti—Cu shape memory alloy electrothermal actuator element, wherein the wire in the actuator element as claimed in claim 2 is subjected to repeating cycles of heating to a temperature of Af or higher with applying a weight load corresponding to 10 to 30% of a breaking load of the wire, and cooling to a temperature of Mf or lower, and wherein the deterioration rate of shape strain recovery after repeating desired times of shape recovery movement is 0.2% or less.
 7. An Ni—Ti—Cu shape memory alloy electrothermal actuator element, wherein the wire in the actuator element as claimed in claim 3 is subjected to repeating cycles of heating to a temperature of Af or higher with applying a weight load corresponding to 10 to 30% of a breaking load of the wire, and cooling to a temperature of Mf or lower, and wherein the deterioration rate of shape strain recovery after repeating desired times of shape recovery movement is 0.2% or less.
 8. An Ni—Ti—Cu shape memory alloy electrothermal actuator element, wherein the wire in the actuator element as claimed in claim 4 is subjected to repeating cycles of heating to a temperature of Af or higher with applying a weight load corresponding to 10 to 30% of a breaking load of the wire, and cooling to a temperature of Mf or lower, and wherein the deterioration rate of shape strain recovery after repeating desired times of shape recovery movement is 0.2% or less. 